1. Field of the Invention
The present invention relates to a multilayer LC filter to be used in mobile communication devices such as cellular phones.
2. Description of the Related Art
Miniaturized, low-loss multilayer LC filters which are suitable for mass production are conventionally used in mobile communication devices such as cellular phones. Such a multilayer LC filter is typically configured in such a manner that a plurality of electrodes are laminated with dielectric layers interposed therebetween. FIG. 6 shows a multilayer LC filter A of such a stacked or laminated configuration. The multilayer LC filter A of FIG. 6 includes a terminal layer c; a ground electrode layer d; an inductor layer e; capacitor layers f, g, h, i, and j; and a ground electrode layer k. These layers are laminated in this order from the bottom side, to thereby form a laminated body a containing a plurality of electrodes. Terminals m, such as an input terminal, an output terminal, and a ground terminal, are formed on the terminal layer c in an exposed condition so that electrical connections can be made thereto.
Further, as shown in FIG. 7, which depicts the assembled filter A, stripe-shaped side-surface or laterally disposed electrodes b are formed on the side surfaces of the laminated body a in such a manner that each of the side-surface electrodes b is connected to a corresponding terminal m. The stripe-shaped side-surface electrodes b extend to the top face of the laminated body a along the direction of lamination, in order to enable reliable mounting of the multilayer LC filter A onto a given mounting surface.
The above-described conventional multilayer LC filter of FIGS. 6 and 7 has frequency characteristics such that no proper or substantial attenuation peak appears on the higher-frequency side of the pass band, although a large attenuation peak appears on the lower-frequency side thereof. Therefore, there is a problem with providing sufficient attenuation on the higher-frequency side. Since the presence or absence of an attenuation peak on the higher-frequency side greatly affects the performance of the filter, a strong demand has arisen for an effective measure that would solve the problem of providing such an attenuation peak.
Conceivably, the above-mentioned problem of insufficient attenuation on the higher-frequency side occurs because the electrodes contained in the laminated body are close to the stripe=shaped side-surface electrodes formed on the outer surface of the laminated body, thereby resulting in the generation of distributed stray capacitance between these electrodes. In particular, stray capacitance is likely to be generated between the side-surface electrodes and each of the ground and inductor electrodes formed in a large area extending to the vicinity of the periphery of the laminated body. In order to solve the above problem, a construction may be employed wherein the distances between the electrodes contained in the laminated body and the side-surface electrodes are increased to a large as possible extent. For example, the dimensions of the laminated body can be maintained unchanged, while the electrodes contained in the laminated body are formed at a central portion of the laminated body such that the pattern area of each electrode is reduced. Alternatively, the pattern area of each electrode can be maintained unchanged, while the dimensions of the laminated body are increased. However, the former measure results in deterioration in the filter characteristics, and the latter measure results in an increase in the physical size of the multilayer LC filter.